1. Field of the Invention
This invention relates generally to collets or chucks for grasping rotating tool bit shafts and, more specifically, to an Improved Collet Assembly and Manufacturing Process.
2. Description of Related Art
FIG. 1 is a perspective view of a conventional dental handpiece 10 within which the device of the present invention might be used. Dental handpieces 10 are used to operate "burrs", or rotating cutting tools, similar to conventional drill or router bits, for cutting or grinding teeth or dental appliances. The typical handpiece 10 is designed to receive compressed air for its operation from a dentist's chair. These handpieces 10 are available in a variety of shapes and sizes, depending upon their purpose and/or application. As depicted, the handpiece 10 comprises a handle 12 and a head 14. A "canister assembly" is housed within the head 14. The "canister assembly" comprises, essentially, a compressed air-NGU powered motor with a removable tool shaft 16 held therein. The tool shaft 16 in a typical handpiece 10 is removable so that different burrs or bits may be used so long as they have the same size tool shaft 16. The tool shaft 16 is held within the canister assembly by the "collet assembly".
FIG. 1a depicts a typical prior art collet assembly 60. The state of the prior art collet for dental handpieces comprises a tapered portion 62 that further has an axial bore 64 drilled into it that is configured to receive a burr or bit. The burr or bit is held in place by a pair of free-floating wedges 66 that ride in slots 68 formed around the circumference of the tapered portion 62. Once a burr or bit is inserted into this prior collet 60, the wedges 66 are biased towards the shaft to hold the shaft in place both axially and rotationally.
There are at two least problems with this prior collet design. First, there are significant manufacturing difficulties in this design. Since the typical burr or bit is approximately 0.06 inches in diameter, the wedges and slots must be extremely small--on the order of 0.01 inches wide and 0.1 inches long. Because of the high speed of rotation of the shaft in operation, the shaft must be balanced to high precision. This, in turn necessitates extremely tight tolerances for the wedges and slots. While these tolerances are attainable, it adds considerable cost to the manufacturing process. What is needed is a collet that is precision balanced, but does not require extraordinarily tight manufacturing tolerances.
The second deficiency with the prior design as shown in FIG. 1 a is the lack of self-centering capability. Because the collet is fairly thin-walled, there is a limit to the number of slots that can be cut through it before its structural integrity is compromised. The prior collet, therefore, can only have the two slots and wedges. The problem with only having two wedges is that the burr or bit is not automatically centered by the collet. As stated above, the high rotational speed of the collet means that any slight misalignment will result in a short service life for the canister and collet (due to wear). What is needed is an improved collet that self-centers the shaft held therein.